1. Field of the Invention:
The invention relates to a positioning device for transport vehicles constrained to travel along a track and which are propelled by a linear motor drive.
2. Description of the Prior Art:
Prior art positioning devices for these vehicles are shown in the journal Elektrie, vol. 24, No. 10, 1970, pp. 358-361. Shown therein are asynchronous linear motors, used to propel track - constrained vehicles in both directions. The inducing primary parts of the linear motors are stationarily placed along the track for the pulse-wise acceleration and positioning of the vehicles. On each vehicle, there is mounted a secondary part in the form of laminated ferromagnetic material on which are placed bars of high electric conductivity spaced as the pole pitch spacing of the stationary primary part. Provided at positioning points where a vehicle is to be brought to a stop with specified accuracy are four switching means which sense different vehicle positions relative to the positioning point. These switching means are operated successively as a function of travel direction and are coupled with a logic circuit in such a manner that upon the vehicle's arrival in the positioning area an actuation of the first two switching means direction information is stored and a travelling field excitation opposing the vehicle direction and travel is sent to the vehicle windings. Upon the additional actuation of the third switch another pulse of travelling field excitation is sent lasting a fraction of the duration of the reverse excitation in order to safely avoid a reversal of the travel direction of the vehicle before it reaches the positioning point. However, the kinetic energy of the vehicle entering the positioning area at the slowest travel velocities must be sufficient to actuate a fourth switching means which actuates a DC field in the primary part which under ideal conditions brings the vehicle to the positioning point without oscillation.
Because in practice the various vehicles will reach the positioning areas at differing speeds because of different cargoes i.e., masses, and different line conditions, an approach without oscillation is not likely because of the above-mentioned pulsed reversed current braking action. The transport vehicles often will overshoot the positioning point with greater or lesser residual velocity and release the first switching means so instead of a DC field braking action a new opposing travelling field excitation occurs. When this happens, the travel direction of the vehicle reverses and all four switching means are again actuated leading to another DC field excitation. If the travel vehicle overshoots the positioning point in the opposite direction again releasing the fourth switching means, a travelling field excitation in the original travel direction takes place. This alternating of travelling field excitation and DC field excitation continues until the oscillating motion of the vehicle comes to rest at the positioning point. The number, magnitude and duration of these oscillations greatly depends on the vehicle mass and the friction of the vehicle on the track.
It is an object of this invention to achieve, while saving switching means, a reduction of the number of field excitation changes and vehicle oscillations even though the masses of the vehicles and the frictional forces may differ greatly.